1. Field of the Invention
The present invention relates to the field of acoustic transducers. More specifically, the present invention relates to capacitive microfabricated ultrasonic transducers.
2. Description of the Related Art
An acoustic transducer is an electronic device used to emit and receive sound waves. Acoustic transducers are used in medical imaging, non-destructive evaluation, and other applications. Ultrasonic transducers are acoustic transducers that operate at higher frequencies. Ultrasonic transducers typically operate at frequencies exceeding 20 kHz.
The most common forms of ultrasonic transducers are piezoelectric transducers. Recently, a different type of ultrasonic transducer, capacitive microfabricated transducers, have been described and fabricated. Such transducers are described by Haller et al. in U.S. Pat. No. 5,619,476 entitled “Electrostatic Ultrasonic Transducer,” issued Apr. 9, 1997, and Ladabaum et al. in U.S. Pat. No. 5,870,351 entitled “Broadband Microfabricated Ultrasonic Transducer and Method of Fabrication,” issued Feb. 9, 1999. These patents describe transducers capable of functioning in a gaseous environment, such as air-coupled transducers. Ladabaum et al, in U.S. Pat. No. 5,894,452 entitled, “Microfabricated Ultrasonic Immersion Transducer,” issued Apr. 13, 1999 describe an immersion transducer (a transducer capable of operating in contact with a liquid medium), and in U.S. Pat. No. 5,982,709 entitled, “Acoustic Transducer and Method of Microfabrication,” issued Nov. 9, 1999 describe improved structures and methods of microfabricating immersion transducers. The basic transduction element described by these patents is a vibrating capacitor. A substrate contains a lower electrode, a thin diaphragm is suspended over said substrate, and a metalization layer serves as an upper electrode. If a DC bias is applied across the lower and upper electrodes, an acoustic wave impinging on the diaphragm will set it in motion, and the variation of electrode separation caused by such motion results in an electrical signal. Conversely, if an AC signal is applied across the biased electrodes, an AC forcing function will set the diaphragm in motion, and this motion emits an acoustic wave in the medium of interest.
It has been realized by the present inventors that the force on the lower (substrate) electrode cannot be ignored. Even though the diaphragm is much more compliant than the substrate and thus moves much more than the substrate when an AC voltage is applied between the biased electrodes, the substrate electrode experiences the same electrical force as the diaphragm electrode. Thus, when transmitting, a microfabricated ultrasonic transducer can launch acoustic waves in the substrate as well as in the medium of interest, even though the particle motion in the substrate is smaller than the particle motion in the fluid medium of interest. Of particular concern is the situation where the substrate has mechanical properties and a geometry such that resonant modes can be excited by the force on the substrate electrode. In these cases, the acoustic activity of the substrate can undermine the performance of the transducer. One specific example is a longitudinal ringing mode that can be excited in a typical silicon substrate wafer. Since the detrimental effects on transducer performance of the forces and motion of the substrate electrode have not been previously addressed, there is the need for an ultrasonic transducer capable of operating with suppressed substrate modes.
While the suppression of modes, matching, and the damping of acoustic energy exists in piezoelectric transducers, the differences between such piezoelectric transducers and microfabricated ultrasonic transducers are so numerous that heretofore suppression of modes, matching and damping was not considered relevant to microfabricated ultrasonic transducers.